The tints and coatings used on spectacle lenses serve a fashion purpose, can
make vision more comfortable, can improve visibility and contrast, and
can protect the eyes from the effects of harmful radiation. This
chapter focuses on the last three purposes, with the emphasis on prescription
spectacle lenses.

When light passes through a spectacle lens, some light is reflected at
the front surface, some is absorbed or scattered by the lens material, and
some is reflected at the lens back surface. The percentage of incident
light that passes completely through the lens for a given wavelength
is the lens transmittance for that wavelength. A plot of transmittance
versus wavelength provides the basic information needed to understand
the properties of a tinted lens.

The average of the transmittance values over a given wavelength range is
the average or mean transmittance of a lens for that wavelength range. However, the
transmittance of a lens also depends on the spectral distribution
of the light incident on it. The transmittance over the visible
spectrum (380 to 780 nm), weighted by the spectral distribution
of daylight and also by the relative sensitivity of the eye under photopic
conditions (the photopic luminosity curve), is often termed luminous transmittance.1 Ultraviolet (UV) and infrared (IR) radiation transmittances are commonly
specified as mean transmittance over a given spectral range.

Most prescription lens tints are specified by their color and a number
that indicates the approximate luminous transmittance of the tint. Light
tints with transmittances of about 75% to 85%, #1 tints, are used as
fashion tints. With transmittances of about 50%, #2 tints are often
too dark for indoor wear and not dark enough to be effective sunglasses. Because
of these limitations, their use is uncommon. Dark, or #3 tints, which
are commonly prescribed as sunglasses, have transmittances
of approximately 20%.

The maximum transmittance of a clear lens is limited by the reflectance
at the lens surfaces. It is not possible to increase the transmittance
of a lens beyond a limiting value of about 92% unless an antireflective
coating is applied. By decreasing reflectance, the antireflective
coating increases transmittance. A high-quality antireflective coating
can decrease the reflectance at each surface of a lens to approximately 0.5%, providing
a total lens transmittance of approximately 99%.

Three procedures are commonly used to tint a lens. Usually, plastic lenses
are tinted by dipping them in a hot, water-soluble dye. This dye penetrates
a uniform distance into the lens surfaces, providing a tint
of uniform color and transmittance. Almost any color is available. By
dipping only a part of the lens in the dye, gradient tints can be provided, or
the lens can be dyed with multiple colors. The tint can be removed
almost completely by boiling the lens in an alcohol solution. Polycarbonate
plastic does not absorb dye. What is tinted is the abrasion-resistant
coating. For the patient, the result is the same.

Normally, glass lenses are tinted by the addition of a chemical compound
to the molten glass. Lens transmittance varies with lens thickness, so
this type of tint should not be used for lenses of high power. A high-minus-power
tinted glass lens is darker at its edge than at its center, and
a high-plus-power lens is darker at its center than at its edge. A
high-plus-power tinted glass lens can be too dark at its center
for its intended use.

Some glass lenses are tinted by the application of a coating to one or
both lens surfaces. These coatings consist of a thin layer of a colored
glass compound or a metal oxide that is applied using a vacuum deposition
process. The coating is of uniform thickness across the lens surface, so
transmittance does not vary with lens power and thickness. The
colored layer is soft, so a thin layer of another compound, usually
magnesium fluoride, is often applied over this first layer to provide
some abrasion resistance. However, even this protective layer is somewhat
soft, so a coating can be abraded easily and is best applied only
to the lens back surface, providing some protection. The magnesium fluoride
protective coating is extremely thin, and interference effects occur
within the coating so that light reflected from the lens surface
is colored, usually red, blue, or purple. This coloring may not be cosmetically
acceptable to some patients.

The silver- or copper-colored mirror coatings found on both glass and plastic
lenses are applied by a vacuum deposition process. When these are
applied to a tinted glass or plastic lens, the result can be extremely
dark sunglasses. For example, a glass sunglass lens with a 20% transmittance
coated with a mirror coating of 40% transmittance will have
a transmittance of 8% (20% × 40%).

Lightly tinted lenses with transmittances of about 80% often are prescribed
for indoor use, for both cosmetic purposes and comfort. For many
years, pink or flesh-colored tints (Fig. 1) have been recommended for patients who complain of glare and discomfort
while working under fluorescent lights. Before the advent of plastic
lenses, glass lenses capable of absorbing UV radiation, especially the
American Optical Corporation's Cruxite (Southbridge, MA), had
a pink tint. Bausch and Lomb's Softlite (Rochester, NY) tint was
pink as well. This glass was used routinely in aphakic lenses to remove
some of the UV radiation that otherwise would reach the retina in aphakic
patients. The color had little to do with the effectiveness of the
lens in this regard. Pink tints began to be associated with many desirable
features, including UV protection, comfort, and glare protection. Actually, any
tint that reduces visible light transmittance performs
the same function. The colors are not strong enough to have any color-filtering
properties. In the current market, lenses that provide complete
UV protection (whether tinted or not) are the best option for the
patient with aphakia.

Lightly tinted lenses perform a useful function by reducing the internal
multiple reflections within a lens. An overhead source that is brighter
than the foreground, such as a fluorescent light, is reflected from
the rear surface of the lens to the front and then back through the
rear surface into the eye (see Fig. 13). Many more internal reflections occur, but after the first three, the
reflections usually are not bright enough to affect vision. This unwanted
glare can reduce the contrast of the foreground and reflect the light
source as a ghost image. Even a light tint attenuates this stray
light because the reflected light rays pass through the tint three times
and are absorbed on each passage. Pink or other lightly colored tints
may have a placebo effect. Also, because the tints greatly reduce the
ring effect (myopic rings) of high-minus power lenses, cosmetic appearance
is improved, if nothing else.

Fig. 13. Weak minus-power lenses form ghost images of bright objects.

Fluorescent lights emit measurable amounts of ultraviolet B (UVB) radiation, the
radiation from 290 to 315 nm that has been associated with the
development of cataracts. Plastic diffusers may absorb some of this
radiation, and “egg-crate” diffusers that block a direct
view of the light source also attenuate it. However, patients exposed
to massed banks of fluorescent lights should have lenses or tints that
absorb UV radiation. UVB protection is automatic with polycarbonate and
most CR-39 (PPG Industries, Pittsburgh, PA) plastic lenses. The clinician
should be aware of the properties of the lenses that he or she
prescribes.

Colors other than pink or brown, such as light grays, blues, and lavenders, have
been popular as fashion tints. Lenses with pronounced coloring
should not be recommended for persons with color deficiencies. Strongly
colored lenses, especially red or blue lenses, may affect traffic
signal visibility, and long-term wear of these lenses may alter color
perception for a considerable time after the lenses are removed.2

Sunglass lens tints are specified by both transmittance and color. Transmittance
of visible light should be such that the brightness of objects
in the visual field are brought into a comfort zone of 350 to 2000 candelas/meters2 (cd/m2),3 where cd/m2 is a unit of luminance. Often, there is a sense of glare when the dominant
luminance in the visual field exceeds this level. Some people prefer
less light, but older patients often can tolerate higher levels. Table 1 lists the luminances that are encountered in various situations in which
sunglasses or tinted lenses might be helpful. Light-colored pavements, most
beaches, and grassy areas have luminances such that a 20% transmittance
sunglass lens can bring them into the comfort zone. Standard
sunglass lenses typically have transmittances of 15% to 25%, and these
sunglasses work well for normal outdoor use and for driving. Shady
areas are attenuated to about 60 cd/m2, but this level is still adequate.

Some activities require darker sunglasses, usually of 8% to 12% transmittance, to
bring luminances into the comfort zone. These include winter
sports, especially those performed at high altitudes, mountain climbing, and
flying above the clouds. Darker sunglasses, with transmittances
of 3% to 5%, also are popular for these activities when it is necessary
to face the bright sky beneath the sun.3 Side-shields or deeply wrapped lens designs should be worn with extremely
dark sunglasses to maintain retinal adaptation at the level required
by the sunglass transmittance. However, extremely dark sunglasses are
not recommended for driving because they blacken shady roadside areas, especially
when the driver is entering the shade from a brighter area. A
sun visor is a better solution. The American National Standards
Institute (ANSI) Z80.3-1996 standard for nonprescription sunglasses1 recommends that tinted lenses with transmittances of less than 8% not
be used for driving.

SUNGLASSES AND VISIBILITY

A patient wearing spectacles loses contrast when viewing a dark object
against a bright sky background. This loss of contrast occurs because
the bright light from the sky is reflected multiple times within the lens
and eventually is superimposed on the light from the foreground object. Sunglasses, or
even a lightly tinted lens, can greatly reduce this
reflected light and increase contrast because the multiple reflections
will be attenuated each time they pass through the lens.

Sunglasses also may be used to maintain night vision after prolonged periods
of exposure to sunlight. Two to 3 hours of sun exposure can delay
both the start of dark adaptation and the time needed to reach the night
vision threshold by several hours.4 Longer exposures over several days result in larger threshold elevations. Sunglasses
with transmittances of less than approximately 15% preserve
night vision, whereas transmittances greater than approximately 35% are
not effective.5,6 Whenever night vision is critical, such as for driving or flying, dark
sunglasses should be worn outdoors during the day to maintain optimal
visual performance at night.

Sunglasses or any tinted lens with a transmittance less than about 80% should
not be used for driving at night. Although tinted lenses solve
the problem of headlight glare by absorbing light from headlights, the
lenses greatly decrease the visibility of objects at the side of the
road (e.g., a pedestrian). When a patient is wearing spectacles and reports headlight
glare, the best solution is an antireflective coating. The coating
decreases the visibility of multiple reflections from the lens surfaces. These
reflections contribute to glare. The cause of much headlight
glare is light scattered by the ocular media. This problem becomes worse
with age. There is no good solution to this problem, and patients
usually must modify their habits so that they are not forced to drive
at night. Glare is worse when an uncorrected refractive error is present. Fine-tuning
the spectacle prescription can minimize the glare problem
at all ages, but the adjustment may be especially appreciated by
younger people.

SUNGLASS LENS COLOR

Sunglass colors should be chosen to avoid compromising the visibility of
traffic signals and affecting color vision. Neutral gray lenses are
best for these purposes because their transmittance is relatively constant
across the visible spectrum (Fig. 2). Consequently, gray is by far the most commonly prescribed sunglass color.

Brown and green lenses are other common colors for sunglass lenses. Brown
lenses, which selectively absorb the blue end of the visible spectrum (Fig. 3), tend to increase the subjective impression of contrast by darkening
the blue sky relative to foreground objects and lowering some of the effects
of blue haze. This effect is pleasing to many users. These lenses
also may be beneficial for certain types of color deficiencies. A patient
with protanomaly or protanopia might benefit from a brown lens
instead of a gray lens because of the relatively high transmittance of
the brown lens in the red end of the spectrum, where the patient is least
sensitive. The color vision of deuteranomalous or deuteranopic patients
may be degraded by such lenses because a brown tint attenuates
green radiation. Green lenses (Fig. 4) provide maximum transmittance in the portion of the visual spectrum, the
green, to which the human eye is most sensitive, while absorbing red
and blue light. However, a green tint should not be used for a patient
with a color deficiency because it narrows the visual spectrum.

Choosing a sunglass lens for a patient with a color deficiency requires
care. The color vision of these patients may be aided by one color of
tint and hindered by another, so it is important to identify the exact
color vision deficit that is present. This task is not simple because
most color vision tests do not classify deficiencies by type but only
indicate whether a color deficiency is present. Exact diagnosis or identification
usually requires an anomaloscope, the Farnsworth-Munsell 100 hue
test, or the Panel D-15 test,7–9 color vision tests that are not available in most clinical practices. Generally, the
gray sunglass lens is the best option for all patients.

A large body of literature shows that UV radiation can have harmful effects
on the eye. The most concern has been for solar UV radiation, but
important industrial or non-natural sources of UV radiation include mercury
vapor lamps, lasers, and welding arcs. There is no evidence that
scattered and reflected IR radiation in daylight is damaging, although
industrial sources of IR radiation may be a cause of eye problems. The
effects of everyday exposure to blue light from the sun are more controversial, but
direct viewing of the sun can damage the retina in a
few seconds.

THE SUN AS A RADIATION SOURCE

The amount of solar radiation that reaches the surface of the earth varies
with a large number of factors, including time of day, latitude, longitude, altitude, cloud
cover, and the amount of ozone, water vapor, and
dust in the atmosphere. Solar irradiance as a function of wavelength
at the earth's surface is often calculated from the irradiance
measured outside the atmosphere, with allowances for these factors.10 An example is shown in Figure 5. The shortest wavelength that reaches the surface of the earth is UV radiation
of wavelength 288 nm. Shorter wavelengths are absorbed by molecular
and atomic nitrogen and oxygen in the atmosphere and by the ozone
layer of the earth.11 UV radiation from 290 to 380 nm is significantly transmitted, with the
transmittance increasing as wavelength increases, and the visible spectrum
from 380 to 780 nm is also highly transmitted. The near IR radiation (or
IR-A) from 780 to 1400 nm reaches the surface of the earth in
large amounts, but longer wavelengths (1400 nm to 1 mm) are heavily absorbed
by molecules in the atmosphere. IR radiation is responsible for
the feeling of heat when the skin is exposed to the sun. The effects
of UV radiation on the skin include tanning, sunburn, aging, and skin
cancers.12

The cornea absorbs all radiation below approximately 300 nm, with transmittance
increasing rapidly above this wavelength (Fig. 6A). Absorbed radiation has the potential to cause damage, and studies of
both animals and humans have shown that the action spectrum for corneal
damage from UV radiation extends from approximately 220 to 310 nm.13 The UVB wavelength band, from 290 to 315 nm, has the shortest wavelengths
and therefore the highest energies of all radiation that reaches the
surface of the earth from the sun. UVB radiation is strongly reflected
by snow, greatly increasing the levels reaching the eye.14 Corneal damage caused by a few hours exposure to high levels of UVB in
this situation is termed snowblindness, actinic keratitis, or photokeratitis.13 A patient with photokeratitis has pain, photophobia, lacrimation, blepharospasm, and
a gritty feeling or a sensation of a foreign body in the
eye, usually 6 to 12 hours after exposure to UV radiation. The skin
of the eyelids and face are reddened. Fluorescein staining of the cornea
shows focal loss of epithelial cells. Symptoms usually disappear within 48 hours, and
permanent damage is rare. Photokeratitis is not commonly
found with other exposures to solar UV radiation because most other
surfaces (e.g., grass, water, sand, concrete) are poor reflectors of UV. In addition, the
eyebrows, eyelids, and eyelashes provide some protection from direct (nonreflected) solar
UV radiation. A prone sunbather could be a victim
of photokeratitis if the sky is viewed for extended periods of time.15 Sunbathers should wear eye protection.

Fig. 6. Transmittances of the human cornea (A) and lens (B) as a function of wavelength. Transmittance at the short-wavelength end
of the spectrum decreases with age. (Modified from Boettner EA, Wolter JR: Transmission of the human ocular
media. Invest Ophthalmol 1:776, 1962.)

When photokeratitis is associated with exposure to a welding arc, the condition
is often termed welder's flash. Electric welding arcs emit large amounts of visible, IR, and UV radiation, including
UVA (315 to 380 nm), UVB, and UVC wavelengths. UVC wavelengths
are shorter than 290 nm and are not part of the solar spectrum. Welders
must be protected from this radiation, and the ANSI Z87.1 standards
for industrial eye protection provide the transmittance requirements
for welding filters.16 The affected individual need not be a welder, but could be an assistant
or bystander who is not wearing eye protection. As with snowblindness, the
signs and symptoms of welder's flash usually disappear within 48 hours.

Pterygia and climatic droplet keratopathy have both been associated with
long-term solar UV exposure,17–22 and at least one study suggests that the development of pterygia and climatic
droplet keratopathy may be associated with long-term exposure
to visible light.23 However, other factors, such as dietary deficiency, low humidity, and
chronic eye irritation from wind and dust, also may be involved.

Lens

The crystalline lens absorbs most UV radiation below 400 nm, acting as
a filter that protects the retina from most UV radiation that is transmitted
by the cornea (see Fig. 6B). Radiation that reaches the lens has been implicated in the development
of cataracts. Animal studies24 indicate that acute exposures to UV radiation from 295 to 320 nm (primarily
UVB) causes lens opacities. UV radiation of longer wavelength (UVA) does
not cause damage, even at high exposure values. More recently, epidemiologic
studies25–29 suggest that chronic UVB radiation exposure is associated with the development
of cortical cataracts but not nuclear cataracts, and that chronic
UVA exposure is not associated with the development of cataracts.

The potential depletion of atmospheric ozone by chlorofluorocarbons may
have an impact on the incidence of cataracts that are associated with
UV radiation. As the ozone layer thins, UV radiation levels are expected
to increase, and the increase is expected to be greatest for UVB radiation.30 These wavelengths are among the most damaging to the lens, so the incidence
of cataracts is expected to increase, as is skin damage related
to exposure to UV radiation.

Retina

When the crystalline lens is removed, large amounts of both UVB and UVA
radiation reach the retina. Animal studies suggest that this radiation
has a low threshold for damage to the retina.31,32 Patient reports of erythropsia after cataract surgery are probably related
to this increased UV exposure,33,34 as is a selective loss of blue cone sensitivity after a number of years.35 The incidence of cystoid macular degeneration after cataract surgery can
be reduced with the use of UV-protective intraocular lenses.36 UV radiation as short as 365 nm, and probably shorter, can be detected
by the aphakic patient and appears violet.37,38 Because of the chromatic aberration of the eye, this radiation is of focus
and decreases image contrast. It is highly recommended that all intraocular
lenses provide protection against UV radiation, and all aphakic
patients without UV-protective intraocular lenses should have complete
UV protection in their spectacles, contact lenses, and sunglasses.

UV radiation reaches the retina of the intact (phakic) eye in small amounts, primarily
in a band transmitted by the lens that extends from approximately 300 to 340 nm, peaking at 320 nm (see Fig. 6B). This radiation is of high energy, and the retina has a low damage threshold, so
concern exists that solar UV exposure may cause retinal damage.39,40 Further research is needed to determine whether chronic low-level exposure
to UV radiation from the sun can damage the retina of the nonaphakic
eye.

Medications and Exposure to Ultraviolet Radiation

A number of medications are known to be photosensitizing; that is, these
medications increase the sensitivity of the skin or the eyes to the
effects of UV exposure. The psoralens are among the best known. Some psoralens
are used in combination with exposure to UVA radiation to treat
dermatologic problems, and cataracts can develop in patients who take
these medications if they are exposed to solar UV radiation during
the treatment period.41 Many other drug types have been reported to be photosensitizing, including
tetracyclines, sulfonamides, phenothiazines, sulfonylurea, allopurinol, and
some oral contraceptives.11,39

EFFECTS OF INFRARED RADIATION

Although it reaches the surface of the earth in large amounts, solar IR
radiation is of low energy relative to UV radiation, and the IR reaching
the eye either directly or by reflection from trees and grass is not
harmful.42 Molten materials are the primary industrial sources of IR radiation. Many
of the industries that produce these materials are automated, and
employees may not be exposed to large amounts of IR radiation. However, direct
ocular damage from IR radiation has been documented for long-term, relatively
high-level exposures in industry. Studies showed that
glass, iron, and steel workers had increased levels of wedge-shaped aging
cataracts relative to control groups.43–45

When protection from IR radiation is desirable, the best protection is
provided by metallic gold, aluminum, silver, copper, and chromium-nickel
alloy coatings that reflect large amounts of incident radiation.42 Dark green glass sunglasses (see Fig. 4) absorb IR radiation, but shorter IR wavelengths may be reradiated at
longer IR wavelengths, providing less protection in industrial situations.42 Plastic lens materials generally do not provide IR protection, nor do
other types of glass lenses.

EFFECTS OF BLUE LIGHT

Blue light has been implicated in damage to the retina (the “blue
light hazard”), and this damage is of most concern for industrial
workers who have chronic exposure.42 Blue light from natural daylight does not reach the eye in high enough
levels to require special protection, although the retinal damage that
occurs with deliberate sun gazing is attributed primarily to photochemical
damage from blue light.14 It has been speculated that chronic exposure to blue light from the sun
may be related to the development of age-related maculopathy and other
eye diseases, and possibly to the progression of retinitis pigmentosa.23,40,46–48 A standard gray sunglass lens reduces the level of blue light that reaches
the retina by approximately 80%. When more complete protection is
considered necessary, then a yellow or brown lens is required. Yellow
and brown tints can alter color perception considerably and may not meet
ANSI Z80.3 standards for recognition of traffic signal color.49

ULTRAVIOLET PROTECTIVE LENSES

Clear Lenses

Ultraviolet transmittance curves for the most common clear ophthalmic lens
materials are shown in Figure 7. Only CR-39 plastic with a dye that absorbs UV radiation, polycarbonate, and
other high-index plastics absorb all UV radiation. CR-39 plastic
without a UV-protective dye absorbs the UVB and a part of the UVA but
does not absorb UVA from 350 to 380 nm. Clear crown glass does not provide
UV protection because it transmits radiation above approximately 290 nm, including
almost all solar UV radiation.

Standard prescription sunglass lenses (gray, brown, and green), whether
crown glass or CR-39 plastic, absorb all UVB radiation and approximately 95% of
the UVA radiation, whereas polycarbonate plastic sunglass lenses
absorb essentially all UV radiation (see Figs. 2 through 4). Partial absorption of UVA is not adequate in some situations. For example, aphakic
individuals without UV-protective intraocular lenses and
people who spend large amounts of time outdoors, especially at low latitudes
or high altitudes and in situations in which there is a large
amount of reflected UV radiation from snow require complete UV protection. When 100% UV
absorption is desired, a protective coating or dye
can be applied to CR-39 plastic or crown glass sunglass lenses.

It is more difficult to make general statements about the UV protection
provided by nonprescription sunglass lenses because the lens materials
are obtained from a large number of sources, both domestic and foreign. Generally, the
UV absorption of these lenses cannot be predicted from
their color or cost.50,51 Inexpensive sunglasses may provide just as much protection as more costly
ones. Any polycarbonate nonprescription sunglass lens can provide
complete UV protection, with the added advantage of extreme impact resistance. Because
of the increased awareness of the need for protection
against UV radiation, nonprescription sunglasses that provide no protection
or poor protection are becoming less common, but they are still
available. Many manufacturers now label their sunglass lenses with UV
transmittance percentages or with statements indicating conformity to
the UV and visible light transmittance requirements of the ANSI Z80.3-19961 nonprescription sunglass standard. There are no current governmental (US
Food and Drug Administration [FDA]) requirements for sunglass
labeling, and some labeling can be misleading. Inexpensive “UV
meters” available for use in the optical dispensary tend to
overestimate the UV transmittance of a lens.52 The best way to determine the UV transmittance of a lens is from its transmittance
curve. Some sunglass manufacturers and importers supply this
information on request.

Even with lenses that absorb all UV radiation, UV can reach the eyes from
the sides and top of sunglasses. Radiation that reaches the eye from
the temporal side may be focused by the cornea onto the nasal limbus
and the nasal side of the crystalline lens.53,54 This phenomenon may account for both the nasal location of most pterygia
and the inferonasal location of many cortical cataracts.55 Large lenses worn close to the face provide better protection, and the
use of side-shields or a deeply wrapped frame further protects the eyes.56 Wearing a hat with a brim also reduces the amount of UV radiation reaching
the eyes.57

Nonprescription sunglasses vary in optical quality. The best lenses are
produced in the same manner as prescription lenses. Sunglass lenses fabricated
from bent plastic sheets may have localized areas of poor optics
that can cause eyestrain or decreased visual acuity. Optical quality
can be evaluated by viewing a patterned surface or an edge (window
blinds, fluorescent lighting fixtures, window or door frame) through
the lens while holding the lens at arm's length and moving the lens
back and forth or up and down. There should be no blurring, localized
bending, or waviness of the pattern as the lens is moved.

Lenses should be dark enough to provide comfortable vision and maintain
night vision. When choosing sunglasses indoors, a good rule of thumb
is that the eyes should not be visible when looking into a mirror. A transmittance
of 15% to 25% is best for most purposes, although darker
tints can be used for special situations (skiing, mountain climbing, flying
above the clouds, at the beach). Extra dark sunglasses should be
a deeply wrapped design or should use side-shields.

Lenses should match in color and transmittance. Gradient tints should extend
equally down the lenses and should lighten gradually.

No wavy areas, localized distortion, or blurring should be noted when viewing
a patterned surface through nonprescription sunglass lenses.

Lenses should not affect color vision or traffic signal visibility. Gray
tints are best for this purpose.

Lenses should be impact resistant. Polycarbonate has the best impact resistance
of all materials, although all sunglasses must meet FDA standards
for impact resistance.

Lenses should absorb UV radiation as required for their purpose. The ANSI
Z80.3 standard for nonprescription sunglasses recommends a maximum
UVB transmittance of 5% for general purpose sunglasses and 1% for special
purpose sunglasses. UVA transmittance should not exceed the visible
light transmittance for general purpose sunglasses and should not exceed
one half of the visible light transmittance for special purpose sunglasses.

Sunglass frames should be large to provide adequate protection. The temple
pieces should not be so large that side vision is affected.

Table 3 provides a partial list of those people who require UV-protective lenses. Aphakic
individuals without UV-protective intraocular lenses require
protection in both their clear prescription lenses and in sunglasses. People
who spend large amounts of time outdoors should have UV protection
in both their clear lenses and their sunglasses. UV protection
is more important in southern climates and in mountainous regions. UV
levels are highest in the middle of the day, from approximately 10:00 AM
to 3:00 PM.14 People who are outdoors at this time of day have a greater need for protection
against UV radiation.

Children who play outside or are exposed to excessive UV radiation, to
delay photochemical responses in the corneal endothelium, lens, and retina

All patients, to maintain healthy eyes and to eliminate, reduce, or delay
the prevalence of corneal problems, cortical senile cataracts, and
solar retinopathies that are induced by UV radiation*

UV, ultraviolet.*This suggestion is made because exposure to UV radiation is cumulative
and it is hoped that protection will prevent radiation-related difficulties
for young people and delay senescent changes in older adults. (Pitts DG: Ultraviolet radiation: When and why. Probl Optom 2:95, 1990)

Photochromic lenses darken or change color when exposed to light or UV
radiation. These are popular lenses, and the two most well-known are probably
the glass Photogray Extra lens from Corning, Inc (Corning, NY) and
the plastic Transitions Gray lens from Transitions Optical (Pinellas
Park, FL). Two recently introduced gray plastic photochromic lenses, Corning
Sunsensors and the Rodenstock USA (Danbury, CT) ColorMatic
Extra, are of similar design and compete directly with Photogray Extra
and Transitions for market share. When worn outdoors on a warm, sunny
day, these photochromic lenses darken to a transmittance of 20% to 25%, a
value that is a comfortable sunglass transmittance for everyday use. Indoors, these
photochromic lenses have a light tint. The lenses do
not darken completely when worn in a car, in part because the lenses
are shaded by the car top, but also because the car windows block some
of the ultraviolet radiation needed for the photochromic reaction. For
this reason, photochromic lenses are not usually recommended for driving
in bright situations. Some patients appreciate them when driving
in suburban areas.Photochromic lenses darken rapidly on exposure to light
but lighten more slowly when returned to the dark (Fig. 8). For this reason, photochromic lenses should be prescribed with care
for people whose work requires that they rapidly travel from outdoors
to indoors (e.g., a forklift driver at a warehouse). The lenses are fine for outdoor use, but
the slow lightening of the lenses may compromise visual function
for the first few minutes of indoor wear.

Transmittance curves for the glass Photogray Extra and the plastic Sunsensors
lenses are shown in Figures 9 and 10. Because both lenses are gray in color, the curves are essentially flat
across the visible spectrum. Both lenses also provide UV protection. Corning
states that their Photogray Extra and Sunsensors lenses each
absorb 97% of the UVA and 100% of the UVB. The manufacturers of the Transitions
Gray lens and ColorMatic Extra lens claim that their lenses
absorb 100% of the UVA and UVB.

Glass and plastic photochromic lenses have some important differences. First, a
plastic photochromic lens weighs much less than a glass lens
of the same power. This characteristic has driven the photochromic lens
market toward the present domination by plastic lenses. Second, some
plastic photochromic lenses are available in high-index materials, including
polycarbonate. Although high-index glass photochromic lenses are
available, they may be too heavy for comfortable wear at the higher
powers at which they are normally used. Third, glass photochromic lenses
require a “break-in” period when first worn. A new Photogray
Extra lens has a light green color that changes to the normal gray
color over the first week or two of wear. During the same time period, lens
performance improves. The lens darkens faster and more on exposure
to sunlight. The need for a break-in period is not a competitive
factor, but patients should be informed about it when they receive their
new lenses. Fourth, the transmittance of a plastic photochromic lens
is more sensitive to ambient temperature than that of a glass photochromic. At
higher temperatures, plastic photochromic lenses do not darken
as much as glass. Glass photochromic lenses tend to perform better
than plastic photochromic lenses in warm climates. Finally, plastic
photochromic lenses slowly wear out over time. After about 2 years of
normal use, a darkened plastic photochromic is a few percent lighter
than when new. Most patients are not affected by this because they probably
will replace their lenses before the change in performance becomes
noticeable.

The large market for photochromic lenses has stimulated the introduction
of a number of specialized photochromic products. The glass Photosun
II and the plastic Transitions Extra Active lenses are darker photochromic
lenses designed to be worn primarily as sunglasses. These lenses
are generally too dark indoors for comfortable wear. Thin and Dark is
a glass photochromic lens that has a 1.5-mm center thickness yet still
meets impact resistance requirements when properly tempered. The lens
is as dark as a Photogray Extra lens when exposed to sunlight. Photogray
II is a glass photochromic lens that does not darken as much as Photogray
Extra. This lens is marketed as a “comfort” lens
rather than as a sunglass lens. Corning's CPF lenses are red, orange, and
yellow glass photochromic lenses that are marketed as providing
protection from the effects of blue light. The characteristics of these
lenses are similar to those of the yellow lenses described in the
next section.

Table 4 lists the lightened and darkened transmittance values for commonly used
photochromic lenses. The transmittance values depend on many variables, the
most important of which is ambient temperature. At higher temperatures, the
lenses do not darken as much, whereas at lower temperatures, the
lenses darken more than normal. For example, the transmittance
of a darkened, 2-mm thick, chemically tempered Photogray Extra lens
is stated to be 22% at 77°F, but the transmittance increases to 39% at 104°F
and decreases to 19% at 32°F. A darkened Transitions
lens has a transmittance of 22% at 72°F, but its transmittance increases
to 30% at 95°F and decreases to 15% at 50°F.

TABLE 4. Commonly Used Photochromic Lenses

Indoor (Lightened) Transmittance (%)*

Outdoor (Darkened) Transmittance (%)*

Glass Photochromics

Photogray Extra(Corning)

85

22

Photobrown Extra(Corning)

85

22

Photosun II(Corning)

40

12

Photogray II(Corning)

89

41

Thin & Darkgray or brown(Corning)

88

16

Photogray Extra 1.6(Corning)

83

22

Photobrown Extra 1.6(Corning)

83

30

CPF 450(Corning)

67

19

CPF 511(Corning)

44

14

CPF 527(Corning)

32

11

CPF 550(Corning)

21

5

Plastic Photochromics

ColorMatic Extra(Rodenstock)

85

16

Sunsensors(Corning)

86

17

Transitions Gray(Transitions)

87

22

Transitions Brown(Transitions)

87

28

Transitions XTRActive(Transitions)

75

15

*Transmittances were measured at 77° F for the Corning glass lenses, 72°F
for the Sunsensors, and Transitions lenses, and 68°F for
the Rodenstock lenses. Transmittance varies with temperature, exposure
history, method of measurement, and (for glass lenses) tempering method
and lens thickness.

YELLOW LENSES

Yellow, red, and orange tints absorb short wavelength light while transmitting
the red and yellow end of the spectrum (Fig. 11). Many patients believe that they see better when wearing these lenses, and
advertising claims include increased visual acuity, increased contrast
sensitivity, and increased stereopsis. Generally, these claims
cannot be substantiated,49 although yellow lenses may improve apparent contrast when skiing.58 Because yellow lenses absorb blue light, beneficial effects have also
been claimed for many ocular pathologies, but at present there is little
evidence for any of these claims. When yellow, red, or orange tints
are worn, color vision may be altered and traffic signal visibility decreased.49

Fig. 11. Transmittance curves for typical glass and CR-39 plastic yellow tints. The
transmittance curve for a red or orange tint would be similar except
that it would be shifted to the right; that is, a red or orange tint
would transmit only longer wavelengths.

Yellow lenses are occasionally advocated for driving at night. However, a
yellow-tinted lens typically has a luminous transmittance of about 80%. A
lens of this transmittance, although it may reduce glare from the
headlights of oncoming cars, also decreases the visibility of objects
along the side of the road, creating a potentially hazardous situation. Yellow
lenses and darkly tinted lenses of any color should never
be worn while driving at night.

MIRROR COATINGS

Mirror coatings (reflecting filters) are created by depositing a thin layer
of metal onto a lens surface in a vacuum. By reflecting most of the
incident light, the coating decreases the amount of light reaching
the eye. Probably the most common coating materials are chromium, aluminum, and
copper. Mirror coatings often are used in combination with other
tints to provide darker lenses than are normally available. These
darker lenses are used in sunglasses for special purposes, such as mountain
climbing or snow skiing.

POLARIZING LENSES

A polarizing lens is the only lens type that actually can reduce glare
from an excessively bright point relative to other objects in the visual
field, if the bright point is caused by a reflection of light from
the sun. Sunlight reflected from any surface, such as the rear window
of a car or from water, is partially or completely horizontally plane-polarized; that
is, the reflected light waves tend to vibrate in the horizontal
plane. A vertically oriented polarizing lens blocks the reflected
polarized light, reducing its brightness. Polarizing lenses are
very popular as sunglasses for driving and fishing and are available in
both prescription and nonprescription form and in both glass and plastic
lens materials.

DIDYMIUM LENSES

When a glassblower or other glass worker heats glass directly in a flame, the
flame surrounding the glass emits a yellow light termed sodium flare. Didymium or neodymium glass (Fig. 12) absorbs this light, which is emitted at approximately 589 nm, making
it easier for the glass worker to view his or her work. The luminous transmittance
of didymium glass is approximately 55%, depending on lens
thickness, and didymium lenses can be worn indoors under normal lighting
conditions with little effect on visual acuity. However, didymium
glass does not provide good protection from UV or IR radiation. It should
only be worn in lower-temperature industrial applications, where radiation
levels are low.

Fig. 12. Transmittance curve for a didymium lens. The absorption band from approximately 570 to 590 nm
absorbs the yellow light of sodium flare.

Didymium lenses have occasionally been advocated for enhancing apparent
contrast. The claims made are similar to those made for yellow lenses. Improvements
in visual performance while wearing these lenses are difficult
to document.

Every optical surface reflects light. The brightness of the reflection
is a function of the index of refraction of the material on each side
of the boundary; the larger the index difference, the more light is reflected. Thus, high-index
lenses reflect more light than do lenses of
lower index, which suggests one use for antireflective coatings. Light
also may be reflected multiple times within the surfaces of a lens, providing
an infinite number of reflected images. Only the first few of
these reflected images may be visible because each reflected image is
dimmer and dimmer.

The most common reports about reflection come from patients wearing a weak
myopic prescription (-0.50 to -1.50 D). The source of the problem
is shown in Figure 13. Light from a bright source, usually an overhead lamp, is reflected from
the ocular surface of the lens to the front surface, then back into
the eyes. This reflection occurs with all lenses, but with a low-minus
power lens, the reflected ghost image is nearly in focus, so it looks
like a double of the original object. It is also a little nearer to
the center of the field than the object, making it especially annoying.

An antireflective coating reduces the visibility of this reflection significantly, but
it does not eliminate the reflection completely. A light
lens tint may decrease the visibility of the reflection because the
light passes through the lens three times before reaching the eye, and
on each pass, the tint attenuates the light. Raising the optical centers
of the lenses moves the image of the ghost upward and may possibly
move the image out of the field of view.

A second type of reflection problem is caused by bright sources such as
street lights, automobile headlights, and windows that are viewed against
an otherwise dark background. These sources are imaged by the cornea
of the eye, which acts as a mirror to form a corneal reflex. This
reflex is then reflected off the front or back surface of the spectacle
lens (Fig. 14). Patients may describe this reflection as a flare or halo or report a
sharply focused spot of light. High-plus and high-minus power lenses
accentuate the problem.

Fig. 14. Bright sources can be imaged by the cornea and then reflected from the
surfaces of a spectacle lens to form an annoying bright spot in the visual
field.

This type of reflection can be difficult to identify. If tilting the lenses
causes the reflection or ghost image to move radically, then the
reflection is from the cornea. Often, a slight change in pantoscopic tilt
eliminates the problem. If the lens is of high plus power, the reflection
is from the front surface of the lens, so a light tint may decrease
its visibility.

As previously mentioned, reports of glare from headlights when driving
at night frequently are related to reflections from a patient's lenses. The
best solution in this case is an antireflective coating, which
both decreases the visibility of reflections and increases the lens
transmittance.

A third type of reflection problem occurs when light sources behind the
patient are reflected from either or both lens surfaces into the eye (Fig. 15). The simplest solution for this annoying reflection is to change the
pantoscopic tilt or face-form angle of the frame (bending the frame at
the bridge). In most cases, this adjustment eliminates the problem. Antireflective
coatings also are effective, but a tint usually does not
help because most of these reflections are from the back surface of the
lens, and the reflected light does not pass through the lens. In addition, a
tint provides a dark background against which the reflections
become more visible. Steepening the base curve of the lenses by 1.00 to 2.00 D
also may help, but this is the last option to be considered
because the lenses must be remade.

Fig. 15. Bright light sources above and behind the eye can be a cause of complaints
about reflections.

For all of the aforementioned types of reflections, if the patient understands
the source of the problem and is assured that nothing is wrong
with his or her vision or the lenses, the concern often is eliminated.

A fourth type of reflection is the myopic ring of strong minus-power spectacle
prescriptions (Fig. 16). This white ring appears to surround the lens at its edge and is caused
by multiple reflections of the lens bevel within the lens. Myopic rings
detract from the cosmetic appearance of the glasses more than they
affect the patient's vision. An antireflective coating is effective
in correcting this problem, as is a light tint.

Fig. 16. The myopic rings of strong minus-power lenses are caused by multiple reflections
of the lens edge within the lens.

Reflections are a common cause of patient complaints. They can be controlled
and reduced, but not eliminated, by the use of antireflective coatings
and light tints. A light tint only decreases the visibility of
reflections that pass through the lens, so the source and type of reflections
should be identified before prescribing. Changes in the pantoscopic
tilt of a patient's glasses also may help by moving the reflection
out of the line of sight. This adjustment is always the simplest
and least expensive method of solving reflection problems, and the
first thing to try.

THE ANTIREFLECTIVE COATING

Light energy moves forward in waves. These waves are analogous to water
waves, with crests and troughs. If a pebble is tossed into a pool of
water, waves radiate outward. If a second pebble is tossed in, where crests
from the two sets of waves meet, there is a higher crest. Where
the crest of a wave from one source meets the trough from the other, the
waves flatten out and disappear.

A thin transparent coating can be deposited on a spectacle lens in a vacuum. If
the thickness of this layer is equal to one fourth of the wavelength
of light (actually slightly less than one fourth of the wavelength
to allow for the decreased velocity of light when traveling through
the coating), and if the index of refraction of the coating is chosen
properly, light waves reflected from the interface between the coating
and the glass are one-half wavelength out of step with the waves reflected
from the outer surface of the coating (Fig. 17). The two wave trains cancel each other (destructive interference), and
no reflections are seen.

Fig. 17. Optical principle of an antireflective coating. When the coating is ¼-wavelength
thick, light waves reflected from the two surfaces of
the coating are exactly ½-wavelength out of phase and cancel (destructive
interference). (Modified from Young JM: AR coating: a definition. Optical World 17[110]:8, May 1988)

The antireflective coating does not destroy light; the interference or
interactions that occur between the different light waves only redistribute
the light. Because less light is reflected, more is transmitted. For
this reason, it is generally not a good idea to apply an antireflective
coating to a sunglass lens. An exception would be a situation in
which reflections from the back surface of a dark sunglass lens irritate
the wearer. An antireflective coating on the back surface of a dark
lens decreases the visibility of these reflections, with only minor
effects on lens transmittance.

In practice, the ideal antireflective coating does not exist. It is difficult
to find a coating of the proper index of refraction, and the single-layer
antireflective coating is of the proper thickness for only
one wavelength of light. Thus, even at the design wavelength, reflections
cannot be completely eliminated, and wavelengths other than the design
wavelength are somewhat reflected. The thickness of a single-layer
coating usually is chosen to be optimal for a wavelength in the middle
of the visible spectrum (in the green), so more red and blue light
than green light is reflected (Fig. 18). Therefore, reflections from a single-layer coating usually are purple, although
the total reflection from the lens surface is decreased.

Fig. 18. Reflectance as a function of wavelength for representative single-layer, bilayer, and
multilayer antireflective coatings. Each surface of an
uncoated crown glass lens has a reflectance of 4.3%. (Modified from Young JM: AR coating: a definition. Optical World 17[110]:8, May 1988.)

Improvements in the technology of coatings now allow multilayer antireflective
coatings for spectacle lenses at a reasonable cost. A number of
extremely thin layers are applied to the lens surfaces in a vacuum, with
the layers alternating in index and thickness. These coatings allow
the reflectance of the lens surfaces to be greatly reduced, and the
reflectance is more equal across the spectrum (see Fig. 18). Reflections from the lens surfaces still are colored. However, the color
depends on the coating design, and the color is more cosmetically
appealing than the purple reflection of the single-layer coating. A high-quality
multilayer antireflective coating can decrease reflections
from approximately 4% at each lens surface to approximately 0.5%, decreasing
the total lens reflectance to 1% and increasing the transmittance
of the lens from 92% to 99%.59

Two difficulties with antireflective coatings are worth mentioning. One, the
coatings are relatively soft and scratch easily. The lenses should
be treated like CR-39 plastic; patients should always wet the lenses
before cleaning and dry the lenses with a soft cloth or facial tissue. The
lenses may be abraded easily if kept in a pocket or purse without
the protection of a case. Second, the lenses get dirty easily. Because
the lenses reflect so little light, water spots, oil smudges, or fingerprints, which
increase reflection, are more visible than for uncoated
lenses.